RBD:Move RBDVolume calls to a separate threads

RBD is a python binding for librados which isn't patched by eventlet.

Time-consuming operations like flattening and copying volumes
blocks eventlet loop and all cinder-volume service hangs
until it finished. It makes cinder-volume services unavailable for
a while.

This patch moves all RBDVolume calls to a separate python thread which
doesn't block eventlet loop.

Change-Id: Id3f2d48428d74011ba690141cee3afad8e48c52f
Closes-Bug: #1658037
(cherry picked from commit 8f2b7f4)

cinder/volume/drivers/rbd.py

@@ -222,6 +222,7 @@ def __init__(self, driver, name, pool=None, snapshot=None,
                                            utils.convert_str(name),
                                            snapshot=snapshot,
                                            read_only=read_only)
+            self.volume = tpool.Proxy(self.volume)
         except driver.rbd.Error:
             LOG.exception(_LE("error opening rbd image %s"), name)
             driver._disconnect_from_rados(client, ioctx)